1. Field of the invention
This invention relates to antenna systems and, more specifically, to conductive-body antenna systems which function effectively at both FM frequencies and AM frequencies utilizing a single discontinuity in the conductive body.
2. Prior art
Much of the prior art in this field is attributable to this inventor. Reference may be had to my U.S. Pat. Nos. 2,923,813, 2,971,191 and 3,007,164 for examples of prior art systems. In general, these and other such systems have been directed to operation in a single band of frequencies, usually lower frequencies, for example the AM broadcast band of the United States which covers the spectral region from 540 to 1600 KHz. With the increasing popularity of FM broadcast reception in automobiles, car-body antennas, such as those available prior to this invention, which operate only in the AM broadcast band, are inadequate. The discontinuities, usually windows, relied upon in such systems for the generation of circulating R-F currents which can be extracted and fed to the antenna terminals of associated radio receiving apparatus, have dimensions which are small with respect to a quarter wavelength at AM broadcast band frequencies (540-1600 KHz) and large with respect to FM broadcast band frequencies (88-108 MHz). Methods for forcing resonance of the discontinuity and its surroundings, in the AM band are described in my patent application, Ser. No. 427,258, filed Dec. 21, 1973 and entitled ANTENNA SYSTEM UTILIZING CURRENTS IN CONDUCTIVE BODY. Such a system has proven very effective in the AM band, but operation in the FM band is not aided by such a system. Fundamentally, I was faced with the problem of the normal discontinuity, or window, in a car body being too small to serve my purposes in the AM band and too large to serve my purposes in the FM band.